Interactions between cell surface components of the early rabbit blastocyst and macromolecules in uterine secretions will be studied, since previous data suggest that these interactions are important in regulating embryonic cell division. The components of these interactions are easily removed from the embryonic surface, and reconstruction experiments will be carried out to determine the sequence of events as the embryonic cells re-enter the cell cycle. Information regarding the embryonic cell cycle and mitotic activity under a variety of experimental conditions will be sought.